During operation of a gas turbine engine, pressurized air from a compressor flows into a head end volume defined within the combustor. The pressurized air flows from the head end volume into an inlet to a corresponding premix passage of a respective fuel nozzle. Fuel is injected into the flow of pressurized air within the premix passage where it mixes with the pressurized air so as to provide a fuel and air mixture to a combustion zone or chamber defined downstream from the fuel nozzle. The fuel and air mixture is burned in the combustion chamber to produce high temperature and high velocity combustion gases. The combustion gases travel from the combustion chamber to an inlet of a turbine portion of the gas turbine engine via a liner or duct that extends at least partially between the combustion chamber and the turbine inlet.
Particular combustion systems may include a bundled tube type fuel nozzle assembly having a plurality of tubes that extend through a forward or upstream plate and through an aft or downstream plate. Each tube extends through a respective opening defined in the aft plate. During operation, cooling air is routed through a gap defined between each tube and the respective opening, thereby providing cooling to a downstream end of the respective tube and to a portion of the aft plate.
Due to various obstructions such as fluid conduits and/or cartridges, it may not be feasible to space the tubes uniformly in all regions of the aft plate. Typically, in order to keep these regions adequately cooled, cooling holes are provided through the aft plate. Cooling air is routed through the cooling holes and into a combustion zone defined downstream from the tubes. However, this cooling scheme may have an undesirable effect on overall emissions performance of the combustor.